Certain external stimuli cause an increase in the free calcium level in the cytoplasm of a frog oocyte. The standard way to measure this is using the biological photoprotein aequorin, which reacts with calcium to emit light. The goal of the project is to be able to measure the relative amounts of calcium produced by various stimulating agents by measuring the number of photons emitted after the stimulus. The emitted light is at an extremely low level-about 50 photons per second--so a very low dark current photomultiplier tube and a cooled housing were used. The oocyte is inserted into a specially-designed stopped-flow cell so the external stimuli can be changed very quickly. Special optics were used to capture as much light as possible and focus it on the photomultiplier tube.